JP2002-171604A, published by the Japan Patent Office in 2002, discloses technology for determining the operating point (rotation speed and torque) of an engine in a hybrid vehicle so as to ensure that a fuel consumption rate per power unit (=fuel consumption amount per power unit) matches a target value. In this prior art, the engine is operated at an operating point at which the fuel consumption rate per power unit matches a preset target value, and the surplus engine power is converted to electric power to charge the battery. In cases where no such operating point exists, the engine is stopped and the vehicle is driven by a motor.
The electric power of the battery is charged while operating the engine at the operating point at which the fuel consumption rate per power unit matches the target value, and hence by determining the engine operating point in the manner described in the prior art, the vehicle is capable of running at the target fuel consumption rate per power unit at all times, whether running by engine while charging the battery or running by motor while discharging the battery.